1. Field of the Invention
The present invention relates to ultraviolet curable coating compositions for cationic electrodeposition which afford anticorrosive properties, such as, waterproofness, chemical resistance, anti-sweating and anti-fingerprint, as well as multi-color variation, to metallic materials including plated products and plastics which are made electrically conductive by plating.
2. Description of the Related Art
As well known in literatures (for example, J. Oil Col. Chem. Assoc., 63, 482(1980)), the electrodeposition is a coating method which comprises dipping an electrically conductive material to be coated into a suspension of a charged film-forming material dispersed in water, and subjecting the conductive material to electrocoagulation by the passage of electric current through the suspension, and carrying out a baking treatment of the conductive material coated with the electrocoaglation.
The electrodeposition method has such major distinctive features that loss of coating material is low; automatic control of coating process is easy to reduce labor costs; a variety of materials to be coated are simultaneously treated; uniform film formation is possible in the inside and edge of the materials to be coated; and coating materials have good adhesiveness to the materials to be coated. Moreover, the electrodeposition using aqueous coating materials has also been appreciated in view of environmental pollution and disaster prevention.
The electrodeposition is classified roughly into anionic electrodeposition and cationic electrodeposition. The cationic electrodeposition has widely been employed as a coating method for the body of cars and in divisions of industrial materials, for the following reasons: since the material to be coated is a cathode in the cationic electrodeposition, metals under the coating or chemical coating film are scarcely flowed out; the coating film resin per se exhibits high anticorrosive properties since it is basic to act as corrosion retarder.
It is a problem, however, that the coating film used in the aforementioned prior art is heat curing and has a thermosetting temperature of 100.degree. C. or higher, and so it cannot be used for heat-sensltive materials such as plastics.
In order to solve this problem, there is a method for coating the materials with ultraviolet curable coating materials by irradiation of ultraviolet radiation, and these coating materials are constituted by ultraviolet curable oligomers, monomers, photopolymerization initiators, sensitizers, and so on. Such coating materials are called coating materials of high-solid or solventless type. In these coating materials, the monomer is used for diluting other components in place of an organic solvent. Accordingly, there arises a problem that splashing of the monomer at the time of coating might afford a bad influence on the human body. This type of coating material has another problem that the coating film produced from this type of coating material is very hard, but fragile and less adhesive to the base material. This is problematic particularly on a smooth plated base material.